The present invention relates to raster-type displays, such as televisions and computer displays, and more particularly to a system for adjusting the timing of vertical control pulses used for controlling the movement or blanking of the electron beam.
Traditional television equipment has been designed to operate from electronic signals that are defined by standards for either broadcast or non-broadcast applications. However, the application of television type equipment to new applications has brought deviations from standards and practice, and have led to difficulty in interfacing products to existing systems. This has been particularly true in interfacing television equipment with computers.
The scan rate for a particular television system, computer display or other raster-type display depends on the number of horizontal scanning lines per frame and the vertical field rate. In this application, the terms "horizontal" and "vertical" as applied to sync pulses, blanking pulses or other control signals are merely orthogonal coordinate references and are not restricted to vertical and horizontal directions in the true sense. Most commonly, the raster beam sweeps across the screen horizontally at a high rate to generate a vertical series of lines of information. However, the beam may sweep the screen as a plurality of vertical lines that are arranged in a horizontal series. Accordingly, for purposes of this application, the "horizontal" or "horizontal rate" sweeps and signals are those occurring at the higher frequency and the "vertical" or "vertical rate" sweeps and signals are those generated at the lower frequency.
Many television systems are interlaced to avoid a visible flicker in a display, usually in a 2:1 ratio. Thus, the 2:1 interlace ratio means that two entire display fields must be developed to generate one frame or complete image.
In circuits used to generate the control signals for the raster, it is convenient to clock the leading and trailing edge of the vertical blanking pulse, that is, the pulse which turns off the electron beam during vertical retrace, from the leading edge of the horizontal blanking pulses. In other words, both the leading edge of the vertical blanking pulse and the trailing edge thereof will be time coincident with the leading edge of horizontal blanking in the even fields. When horizontal and vertical blanking pulses are mixed to form a composite vertical blanking pulse in the even fields, a horizontal blanking pulse occurs at the same time as the non-composite vertical blanking pulse terminates and the effect is to stretch the non-composite vertical blanking pulse by the width of one horizontal blanking pulse to produce the desired composite vertical blanking pulse width. In the odd fields, on the other hand, the trailing edge of the non-composite vertical blanking pulse occurs between horizontal blanking pulses and is, therefore, shorter in duration and terminates earlier in the odd frames than in the even frames.
The uneven widths of the composite vertical blanking pulses may result in jitter of the image on the screen, and results in inaccurate digitizing of the analog signal. Both of these problems are significant in computer displays wherein very accurate rendering of the analog or digital signal is important.
In the cases of industrial and military format and in many computer displays, a composite synchronizing signal is used to drive the deflection circuits and may also be used possibly to control the digitizing of the analog signal. In some systems the synchronizing signals may be generated from the blanking signals. Uneven blanking or synchronizing signals have not typically been a problem in most television receivers used in the broadcast mode because they typically are provided with an AFC network so that if the synchronizing signals are lost, the horizontal oscillator will continue to operate so that the beam will not produce a bar or pinpoint in the event that vertical or horizontal sweep collapses. In many computer displays and other non-broadcast systems, however, the synchronizing signals and blanking signals are used directly to control the sweep of the beam, and any nonuniformities, such as differences in the widths of the vertical blanking pulses or vertical sync pulses, cause jitter and other inaccuracies in digitizing the analog signal discussed above.